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Upgrade nowFuels Used in Hot Water Systems
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Module Lessons
Hot Water Systems: Planning & Design
Fuels Used in Hot Water Systems
Open Vented Storage Systems
Unvented Storage Systems
Localised Water Heaters
Unvented System Pressures
Secondary Circulation Systems
Hot Water Safety Controls
Functional Controls: Strainers & PRVs
Expansion Devices & Check Valves
Pressure Relief, Tundish & Composite Valves
Discharge Pipework: Sizing & Termination
Design Information Sources
Sizing Cisterns & Storage Vessels
Hot Water Pipework Sizing
Sizing Circulation & Boosting Pumps
Sizing Expansion Vessels & Presenting Designs
Installing Unvented Hot Water Systems
Testing & Commissioning Hot Water Systems
Diagnosing Hot Water System Faults
Component Fault-Finding: Pumps, Vessels & Thermostats
Servicing & Maintaining Hot Water Systems
Lesson Overview
Domestic hot water systems utilise fossil fuels, electricity, and renewable energy sources to satisfy consumer demand. Selecting the correct fuel depends on network availability, storage requirements, operating costs, and carbon emission targets.
Learning Objectives
Distinguish between the characteristics and operational properties of fossil fuels and renewable energy sources used for water heating.
Compare the storage, handling, and environmental requirements of liquefied petroleum gas (LPG), fuel oil, and solid fuels.
Evaluate key selection criteria, including geographic fuel availability, appliance design, and smoke control legislation.
Fossil Fuel and Electrical Sources
Natural gas (methane, CH4) remains the most prevalent domestic fuel in the UK, delivering low carbon emissions relative to other fossil fuels and requiring no on-site storage. Where properties lie off the gas grid, alternative fuels must be selected.
| Fuel Type | Boiling Point | Specific Gravity (Air = 1.0) | Key Storage / Operational Property |
|---|---|---|---|
| Natural Gas | -162°C | 0.6 to 0.8 (lighter) | Piped directly via mains infrastructure |
| Propane (LPG) | -42°C | 1.53 (heavier) | External bulk tanks or 47 kg cylinders |
| Butane (LPG) | -4°C | 2.00 (heavier) | Indoor storage only; unsuited to winter cold |
| Kerosene (C2) | 150°C to 300°C | Liquid fuel | 28-second viscosity oil to BS 2869:2017 |
The total amount of heat energy released when a unit volume or unit mass of fuel undergoes complete combustion, measured in MJ/m3 or MJ/kg.
Safety Hazard
LPG is denser than air and pools at low level in the event of a leak. Storage vessels must be positioned away from cellar vents, drains, and unsealed gullies.
Solid Fuel, Biomass, and Renewable Technologies
Solid fuels include coal, anthracite, and manufactured smokeless fuels. Under the Clean Air Act 1993, only authorised smokeless fuels or exempt appliances may be used in designated smoke control areas.
Biomass (wood pellets, wood chips, and logs) is deemed carbon-neutral because the carbon dioxide released during combustion equals the volume absorbed during the tree's growth cycle. Pellets swell and break down if exposed to moisture, requiring strictly dry, well-ventilated storage.
A secondary heat-producing appliance, such as a gas boiler or electric immersion heater, used to supplement renewable systems during peak demands or low ambient temperatures.
Exam Tip
Twin-coil cylinders connect solar primary fluid to the lower coil to pre-heat cold water, while the boiler connects to the upper coil to maintain water above 60°C.
Key Takeaways
Propane boils at -42°C, making it suitable for external winter storage, whereas butane boils at -4°C and fails to vaporise in sub-zero temperatures.
Domestic heating oil must conform to BS 2869 Class C2 (28-second kerosene) for pressure jet burners and low-level balanced flues.
Biomass fuel is classified as carbon neutral, but bulk storage requires substantial, dry, and fire-separated fuel containment facilities.
Renewable systems such as solar thermal and heat pumps require an auxiliary heat source to guarantee required hot water temperatures year-round.
Next Steps
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